Aqueous alkali metal hydroxide solution i.e., caustic is used for removal of various toxicants like sulphides, mercaptans, amines, naphthenic acids, phenols etc. from gaseous and hydrocarbon streams in oil refinery processes. Once these contaminants come in contact and react with caustic solution, it cannot be further utilized and is known as spent caustic. A typical spent caustic may contain about 3-12% of the NaOH with varying but significant quantities of toxic compounds like sulphides, mercaptans, amines, naphthenic acids, phenols, their derivatives, hydrocarbons and few other inorganic and organic compounds. Owing to presence of these contaminants and high salinity and high pH, spent caustics are most difficult of all industrial wastes to dispose properly. Spent caustic is disposed by very expensive and environmentally reactive methods such as high dilutions and then treatment at ETP, deep well injections, incineration, wet air oxidation, humid hydrogen peroxide oxidation etc.
In context of above, biological treatment of spent caustics at atmospheric pressures and temperature would be cheaper and environment friendly alternative to the currently employed treatment methods.
A biological process for the treatment of spent caustics was described by Rajganesh, Sublette, Camp and Richardson, Biotechnology Progress, 1995 (11), 228-230. In this process, sulfides are completely oxidized to sulfate by Thiobacillus denitrificans. This paper discloses a process for treatment of spent caustic wherein only sulfides are removed from the spent caustic. Also, the process is required to be performed at low pH of 7.0 wherein said pH is maintained by addition of acid like 10N nitric acid solution. The major drawback of above said process is that only one impurity like sulfides are treated and therefore, additional process is required for the treatment of other impurities like phenols, amines, naphthenic acids, hydrocarbons etc. from spent caustic. Secondly, an additional step of maintaining the pH by addition of 10N nitric acid solution is employed in the process which results into extra cost in terms of chemical requirements for treatment of spent caustic. Moreover, the process includes use of undefined mixture of microbes which are difficult to replicate and takes long time for acclimatization.
U.S. Pat. No. 5,480,550 discloses a biological process for the disposal for caustic waste streams containing inorganic sulfides to effect neutralization of the caustic and oxidation of sulfides to sulfates. The process disclosed in above said patent includes use of flocculated cultures of a sulfide-oxidizing bacterium from the genus Thiobacillus and various heterotrophs. The process includes immobilization of the bacteria and use of undefined mixture of microbes which are difficult to replicate. This process suffers from major drawback of immobilizing bacteria by co-culture with at least one floc-forming heterotroph under aerobic conditions to form a flocculated biomass. Secondly, the process of disposal of caustic waste streams is performed at pH of 7.0 and same is maintained by addition of 10N nitric acid solution. Hence, addition of heterotroph and nitric acid results into undesired addition of cost to the disposal process.
U.S. Pat. No. 6,045,695 discloses a process for the biological treatment of spent caustic solution containing sulfides, wherein the solution introduces into an aerobic reactor containing sulfide-oxidizing bacteria, and the sulfides are partly converted to elemental sulfur and partly to sulfate by controlling the redox potential in the reactor at a value below 300 mV (against an Ag/AgCl reference electrode), or below −97 (against reference electrode). The process suffers from disadvantages like the sulfide oxidation is done by using M. sulfidovorans which result into formation of thiosulfate. Usually thiosulfate is an undesirable component in waste water. Therefore, it is preferred then to combine the use of M. sulfidovorans with bacteria like genus Thiobacillus to convert thiosulfate to sulfate and/or sulfur which ultimately results into increase in the cost of disposal of impurities from spent caustic.
US patent application 2001/0024351 discloses a method and apparatus for biologically treating a spent caustic to provide a treated spent caustic, said method comprising the steps of: (a) passing a spent caustic stream comprising water, alkali metal hydroxide and sulfide to a first bioreactor; (b) biologically oxidizing sulfide in the first bioreactor with sulfide-oxidizing bacteria like thiobacillus and thiomicrospira) to form sulfur and sulfate to provide a partially oxidized spent caustic; (c) passing the partially oxidized spent caustic to a second bioreactor where at least a portion of the partially oxidized spent caustic is further oxidized with sulfide-oxidizing bacteria to generate sulfate from sulfur to provide a treated spent caustic comprising sulfate.
The drawbacks of above said process is that the process requires two bioreactors for treatment of spent caustic resulting into increase in cost. Further additional step of maintaining the pH to 8.5 is required wherein said pH is maintained by addition of acids like hydrochloric acid or sulfuric acid and hence results additional cost for chemical requirements. Moreover, only one impurity is treated like sulfides and therefore, additional process is required for the treatment of other impurities like phenols, amines, naphthenic acids, hydrocarbons etc. from spent caustic.
Pradeep et al. (2015; Appl. water Sci., Vol. 5: 105-112) discloses biological degradation of phenols from waste water using bacteria like Pseudomonas and B. subtilis. This publication mentions that the optimum pH for biological treatment of phenol is in between 6 to 9. This shows that critical parameters of pH are required to be maintained to perform the degradation process. Here, only one impurity is treated like phenols and therefore, additional process is required for the treatment of other impurities like phenols, amines, naphthenic acids, hydrocarbons etc. from spent caustic.
Therefore, there is a need for an improved method for treating spent caustic which meet effluent discharge standards, and which include minimal step for disposal of all the impurities/contaminants from spent caustic under ambient conditions of pH, temperature and pressure in an effective and economical way. Moreover, such treated spent caustic is required to be re-usable.